(1) Field of the Invention
The present invention relates to an electroconductive composite fiber and a process for the preparation thereof.
(2) Description of the Related Art
Thermoplastic resins such as polyethylene, polyamides and polyesters are used as fibrous products in various fields. However, fibrous products of these thermoplastic resins are defective in that the antistatic property is poor and they are easily chargeable. Troubles caused by static electricity have been discussed, and recently, the problem of static electricity has been particularly commented on. The reason is that recent advance of research on static electricity has revealed that many troubles which have been considered to arise from unknown causes, such as fires and explosions, are due to static electricity and troubles caused by static electricity increase in semiconductors and computers comprising semiconductors.
Increase of such troubles is due to the fact that materials that are easily charged, for example, synthetic fibers and plastics, are increasing around us, because of development of air-conditioning systems, the environmental humidity is reduced and operations are often conducted under a low humidity where static electricity is readily generated, and recently developed OA devices are readily damaged by static electricity. For example, since a cloth formed of polyethylene terephthalate fibers is statically charged during wearing to twine and tangle around the body and render walking difficult. Furthermore, such a cloth absorbs dusts floating in air and becomes dirty, and in case of a dust-free garment, mesh clogging is readily caused. Moreover, a discharge shock is generated when a person walking on a carpet touches a handle of a door, and in this case, if a combustible liquid or gas is present in the vicinity, there is a risk of a fire or explosion. As means for solving these problems, various methods using electroconductive fibers have been proposed.
According to the first method, an electroconductive substance is coated on the surface of a fiber. More specifically, a metal-plated fiber formed by chemically plating a metal on a fiber and an electroconductive fiber formed by coating an electroconductive powder such as a metal powder or carbon black on the surface of a fiber have been proposed. In these electroconductive fibers, the electroconductivity is good at the initial stage, but the abrasion resistance during wearing is poor, and the electroconductive layer present on the surface is peeled by washing and the electroconductivity is accordingly drastically reduced. Furthermore, the chemical resistance is poor and when the fiber of this type is used for a dustfree garment, the garment becomes a dust-forming source.
According to the second method, a composite fiber is prepared by forming a sheath layer of a fiber-forming copolymer around a core of a thermoplastic resin having a powder of an electroconductive substance dispersed therein. In case of an electroconductive composite fiber having electroconductive carbon incorporated therein, since carbon is black, if the sheath layer is thin, the fiber is seen black and cannot be used in the field where an aesthetic effect is important. As means for obviating this disadvantage, there can be mentioned a method in which the amount of titanium oxide in the sheath polymer is greatly increased and incident and refracted light in the sheath polymer is reflected on the surface of titanium oxide, whereby the hue is improved to a grey level. In order for titanium oxide to sufficiently exert an effect of hiding carbon black, a certain distance should be present between the surface of the sheath layer and the core and the core should be present substantially at the center of the section.
Even in the case where a sheath-core composite fiber is formed by using a white electroconductive metal compound such as stannic oxide, if the core is not completely covered by the sheath layer, the electroconductive agent present in the core is decomposed especially by oxidation-reduction chemicals, resulting in occurrence of troubles such as reduction of the electroconductivity and reduction of the performance by falling during wearing. However, if complete covering is attained by the sheath layer, the following electric problem arises.
Although the electroconductivity is good between the sections, since the sheath layer is formed of a polymer having a good fiber-forming property and is electrically insulating, the electric resistance of the surface is high and the electroconductivity of the surface is insufficient.
Accordingly, even in a fabric composed of such a sheath-core type composite fiber containing an electroconductive substance in the core, static electricity is accumulated and the electricity-removing function based on corona discharge by the electroconductive fiber is not properly exerted, but such troubles as twining of a cloth around the body, generation of cracking discharge sounds and adhesion of dusts arise and there is still present a risk of a fire or explosion by static electricity. As means for solving these problems involved in sheath-core composite fibers, Japanese Unexamined Patent Publication No. 60-110920 proposes a method in which the core is eccentrically arranged and the thickness of the sheath layer is controlled below 3 .mu.m. However, this method is defective in that spinning is very difficult, the electric resistance cannot be reduced to a desirable level and deviation of the electroconductivity is large.